It’s been an interesting week at Calvi in northwest Corsica weather wise. In the heatwave that’s been going on in that part of the world, there’s been a constant battle going on between the sea breeze from the north, and the foehn wind from over the mountains to the south (fig 3). The effect of the sea breezes arrival must be very noticeable at times, and must come as a welcome relief to the town (fig 1). It looks like the flip-flop between the two can happen at anytime of the day judging by the plot grid (fig 2), and the land breeze – sea breeze, must obviously be very finely balanced. Yesterday evening for example, the sea breeze which had set in earlier was quickly replaced by a land breeze that kicked in at 22 UTC and increasing the temperature from 26.3 to 31.6°C. The land breeze then failed early this morning at 02 UTC, and the temperature dropped again from 32.5 to 27.8°C, eventually the land breeze set back in at 10 UTC this morning as daytime heating cancelled out the sea breeze. I have marked out some other sharp fluctuations in the plot grid (fig 2).
The observing station at Calvi is located at the airport, in a valley a few kilometres to the southeast of the town itself (fig 3), with the Mediterranean sea to the north, and ringed by high ground to the south, east and west (fig 3).
I notice that the sea breeze has finally relented on the Moray coast, well at least at Lossiemouth, where the temperature has just leapt up over 7°C in just a single hour to 28.3°C at 14 UTC, and in so doing so became the hottest place in WMO block 03 (fig 1). This is so reminiscent of the May Day back in 1990 when the identical thing happened and set the warmest May Day record for the UK in the process. How the sea breeze has been holding back the land breeze and a southerly gradient of at least 20 or 25 knots till now beats me. It happened yesterday, but the gradient wasn’t just powerful enough to win out.
The remarkable thing that happened in the last hour was that as Lossiemouth lost the sea breeze and the temperature leapt up 7.1°C (fig 2), the sea breeze re-established itself just a few miles to the west at Kinloss, and the temperature fell by 4.4°C.
I remember this particular May Day 1990 very well, because at the time I was an observer at Kinloss and was due on the night shift, taking over from John Sutherland who had been on the day shift. He told me the story of how a southerly foehn had kicked in during the early afternoon and increased the temperature by at least 10°C. All morning rotors from the strong southerly wind at 2000 feet, coupled with a sea breeze effect, had been keeping the surface wind in the north or northeast, blowing of a cold Moray Firth (fig 1), but eventually the southerly burst through and the temperatures soared from around 15°C at 12 UTC to around 27.2°C within the hour. I can’t be too precise about the exact times or magnitude in the rise of the air temperature, suffice it to say that John was not particularly happy about getting a phone call from Bracknell asking him to correct his 13 UTC observation! I think the maximum that afternoon which had been a smidgen higher at nearby Lossiemouth at 27.4°C (fig 3), will still rank as one of the warmest May Days on record in Scotland, if not the UK.
One day I’ll have to pay the archives a visit in Exeter, dig out the old obs book if they have it, and jog my memory! I only have six hourly SYNOP reports for that particular day, but here are the 12 and the 18 UTC plotted charts for posterity and for John, who was quite a character.
It looked like the cloud would continue to thin and break, as it had done for most of this morning down here in mid-Devon, and then around 1 PM the sea breeze set in, and low stratus quickly followed to spoil it all. I just wonder if the drier air that’s advecting N’NW from France will clear the low stratus and sea fog in the Channel overnight? The streamlines seem to suggest that it might, as long as the gradient is maintained (fig 2).
Meanwhile on the north coast of Devon, Chivenor at 15 UTC is the warmest place in the country (fig 3).